Mention defaults for forbid option in docs

Listing the defaults in the documentation for these rules makes it
clearer how they work.

Author: lencioni

docs/rules/forbid-component-props.md

@@ -41,4 +41,4 @@ The following patterns are not considered warnings:
 
 ### `forbid`
 
-An array of strings, with the names of props that are forbidden.
+An array of strings, with the names of props that are forbidden. The default value of this option is `['className', 'style']`.

docs/rules/forbid-prop-types.md

@@ -50,7 +50,7 @@ class Component extends React.Component {
 
 ### `forbid`
 
-An array of strings, with the names of `React.PropTypes` keys that are forbidden.
+An array of strings, with the names of `React.PropTypes` keys that are forbidden. The default value for this option is `['any', 'array', 'object']`.
 
 ## When not to use